To meet the demand for wireless data traffic having increased since deployment of 4G communication systems, efforts have been made to develop an improved 5G or pre-5G communication system. Therefore, the 5G or pre-5G communication system is also called a ‘Beyond 4G Network’ or a ‘Post LTE System’.
The 5G communication system is considered to be implemented in higher frequency (mmWave) bands, e.g., 60 GHz bands, so as to accomplish higher data rates. To decrease propagation loss of the radio waves and increase the transmission distance, the beamforming, massive multiple-input multiple-output (MIMO), Full Dimensional MIMO (FD-MIMO), array antenna, an analog beam forming, large scale antenna techniques are discussed in 5G communication systems.
In addition, in 5G communication systems, development for system network improvement is under way based on advanced small cells, cloud Radio Access Networks (RANs), ultra-dense networks, device-to-device (D2D) communication, wireless backhaul, moving network, cooperative communication, Coordinated Multi-Points (CoMP), reception-end interference cancellation and the like.
In the 5G system, Hybrid FSK and QAM Modulation (FQAM) and sliding window superposition coding (SWSC) as an advanced coding modulation (ACM), and filter bank multi carrier(FBMC), non-orthogonal multiple access(NOMA), and sparse code multiple access (SCMA) as an advanced access technology have been developed.
In a multicarrier system, e.g. an Orthogonal Frequency Division Multiplexing (OFDM) system, the frequency tones or subcarriers are orthogonal among each other. Accordingly, the signals do not undergo inter-subcarrier interference in a complete synchronization state. In the Cyclic Prefix OFDM (CP-OFDM) technology in use for the Long Term Evolution (LTE) system, the subcarriers are configured to be orthogonal among each other. Such orthogonality simplifies system design and reduces complexity collocation.
Meanwhile, the next generation, i.e., 5G, LTE technology is focused on multicarrier systems, which may not insist on orthogonality. For example, the Filter-Bank Multi-Carrier (FBMC) system is characterized by modulating signals with filters and negating the orthogonality of subcarriers for enhanced spectral efficiency and spectrum confinement. Negating Cyclic Prefix (CP) and orthogonality can improve spectrum efficiency and diminish sensitivity to time and frequency offsets. The data are modulated by frequency subcarrier (e.g., M subcarriers), pass selected filter banks, and then Fourier-transformed. The length of an FBMC symbol is increased by L multiples of the original length by the filters through which the subcarriers pass. Accordingly, the length of an FBMC symbol, N, is equal to LM (N=LM) in the time domain. In the time domain, consecutive FBMC symbols are overlapped and summed. At this time, the number of symbols being overlapped may be determined depending on the modulation scheme. However, non-existence of frequency domain orthogonality may cause inter-subcarrier interference between neighboring cells using the same frequency subcarriers. Such inter-subcarrier interference may affect all functions of the FBMC system, e.g. channel estimation, synchronization, and equalization. In this case, the number of overlapped symbols may be equivalent to the number of interferer symbols. The FBMC symbols may be interfered with by the interferer symbols on respective frequency components.
FIG. 1 is a diagram illustrating an exemplary cooperative multipoint scenario, FIG. 2 is a diagram illustrating per-cell Cell-specific Reference Signal (CRS) resource mappings in an LTE system, FIG. 3 is a diagram illustrating CRS mapping in an LTE system, FIG. 4 is a diagram illustrating per-cell CRS resource mappings under assumption of FBMC, and FIG. 5 is a diagram illustrating an exemplary CRS resource mapping in a conventional technology under assumption of FBMC.
FIG. 1 shows a multi-cell environment. An access point or a base station (evolved Node B (eNB)) may communicate signals with terminals in cooperation at various levels. Such a scenario may include Coordinated Multi-Point (CoMP) in an LTE-advanced environment. In the case that all cells are cooperating among each other, it may be possible to perform joint transmission in which the terminal can receive signals from a plurality of access points. In a low cooperative situation, the terminal may switch between cells seamlessly through a dynamic cell selection operation. By performing user scheduling and beamforming determination in a cooperative manner, the UE may receive data through only the serving cells. At this time, signals from neighboring cells may act as interference that decreases cooperation gain.
For cooperative operation, channel information must be provided. Particularly in the LTE standard, CRS is used for channel quality measurement, rank-adaptive multiplexing, and closed loop and open loop multiplexing recommendation. In this case, the CRS is transmitted in a frequency-shift manner according to cell IDs of neighboring cells.
In reference to FIGS. 2 and 3, per-cell CRS resources are marked in the time-frequency domain. As shown in FIG. 2, the CRS 215 for the first cell (or cell 1) is mapped to the time-frequency resources as denoted by reference number 210. The CRS 225 for the second cell (or cell 2) and the CRS 235 for the third cell (or cell 3) are mapped to the time-frequency resources as denoted respectively by reference numbers 220 and 230. At this time, the CRSs of the neighboring cells are mapped without being overlapped as shown in the drawing. For example, the CRS 225 for the second cell is mapped to the resources in the state of being shifted on the frequency axis in comparison with the CRS 215 for the first cell and the CRS 235 for the third cell. Similarly, the CRS 235 for the third cell is mapped to the resource in the state of being shifted on the frequency axis in comparison with the CRS 215 for the first cell and the CRS 225 for the second cell. As a consequence, the per-cell CRSs are mapped in the frequency-time domain as denoted by reference number 350 in FIG. 3.
However, if the frequency shift in the RS pattern of the LTE system is applied to the FBMC system without modification, this may cause unsatisfactory channel estimation because of inter-subcarrier interference as described above. At this time, accurate interference may follow accurate design of the filter bank. Since the interference from the closest distance is the strongest interference, the symbol preceding another symbol in a multipath channel situation may undergo stronger interference. In reference to FIGS. 4 and 5, the signals in the respective resource blocks (RBs) may undergo interference caused by neighboring symbols in the frequency-time domain. For example, the CRS 415 for the first cell may be mapped to the resources as denoted by reference number 410 in FIG. 4. At this time, since no orthogonality is guaranteed in the FBMC system, the CRS may cause interference to the signals mapped to the neighboring resource elements as denoted by reference number 417. Likewise, the CRS 425 for the second cell and the CRS 435 for the third cell may cause interference to the signals mapped to the neighboring elements(427.437) as denoted respectively by reference numbers 420 and 430. As a consequence, if the CRS 455 is mapped to the resource in the state of being shifted in the multi-cell environment according to the conventional technology as denoted by reference number 450 of FIG. 5, the signals from the neighboring cells may act as interferences as denoted by reference number 457.
Even in the channel estimation method considering inter-subcarrier interference, e.g., pair-of-pilots method, interference approximation method (IAM), and interference pre-subtraction technique, only the intra-cell interference on the same channel is considered. This cannot be applied for multi-cell interference.